Small information devices such as mobile phones and PDAs (Personal Digital Assistants) have become increasingly sophisticated in recent years and have become capable of communicating with a network or storing a large amount of personal information. Under such circumstances, a demand for improvement in the security performance in such devices is increasing significantly. In order to assure the security in such devices, personal authentication based on a password, an ID (Identification) card and so forth conventionally and commonly used can be adopted. However, the password and the ID card have a high degree of possibility of misappropriation. Therefore, it is much demanded to achieve personal authentication (authentication for certifying that a user of a device is the one registered in advance) having a high degree of reliability.
Personal authentication based on organism information (biometrics information) has a high degree of reliability. Therefore, it is considered that such personal authentication can satisfy the demand described above. Particularly, if a fingerprint is used as the organism information, not only the reliability but also the convenience is high.
If personal authentication is performed using a fingerprint as organism information, a capacitance type fingerprint sensor or an optical type fingerprint sensor is used to sample a fingerprint (a pattern formed from ridges which may contact with a sampling face of the fingerprint sensor and valley lines which do not contact with the sampling face) as image information from a finger of a person to be authenticated. Thereafter, characteristic information (e.g., position information of a bifurcation or an end point) is extracted from a foreground of the fingerprint image (e.g., a ridge image). Then, the extracted characteristic information and registration characteristic information of the person to be authenticated registered in advance are collated with each other. Thus, determination on whether or not the person to be authenticated is the original person, that is, the personal authentication is performed.
Incidentally, a general fingerprint sensor (hereinafter, sometimes referred to as “plane type fingerprint sensor”) for sampling a fingerprint image from a person to be authenticated normally has a sensor face (sampling face) having a size greater than that of a finger. However, in recent years, in order to incorporate a fingerprint sensor into a small-size information device such as a mobile phone set and a PDA, such a method as described below is utilized. That is, the size of the sensor face is formed smaller than that of a finger and a plurality of partial images successively sampled through the sensor face are synthesized to obtain an image of the entire fingerprint.
As a fingerprint sensor which can deal with such a situation, a sweep type fingerprint sensor is available. The sweep type fingerprint sensor has a rectangular sampling face (sensor face/image pickup face) having a length and an area sufficiently smaller than those of a finger. When the sweep type fingerprint sensor is used, a plurality of partial images of a fingerprint are successively sampled by the fingerprint sensor while the finger is moved with respect to the sampling face (fingerprint sensor) is moved with respect to the finger. Then, the entire fingerprint image is reconstructed based on the plurality of sampled partial images. Then, information of a characteristic point (a bifurcation or an end point of a ridge) is extracted and produced from the reconstructed entire fingerprint image, and personal authentication is performed based on the extracted information. It is to be noted that, relative movement of a finger with respect to a sampling face is hereafter referred to as “sweep” or “slide”. Further, there is known a technique in which a sweep type fingerprint sensor is used in a pointing device.    Patent Document 1: Japanese Laid-Open Patent Publication No. 10-091769    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-208620    Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-107366    Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-248820    Patent Document 5: Japanese Laid-Open Patent Publication No. 2001-307078
However, in the technique of the sweep type fingerprint sensor, an overlapping area between the partial images is essential in order to acquire information of the entire fingerprint from the partial images of a sampled fingerprint. Similarly, in the technique of the sweep type fingerprint sensor used in a pointing device, an overlapping area between the partial images is essential in order to measure the movement direction and orientation of a finger.